<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
        <title>Title</title>
    </head>
    <body>
        <h1 >0</h1 >
        <h2>0</h2>
        <script>
            let count=0;
            //开启定时器  每个一秒调用一次方法
            //方法作为参数传递时,不写()
            setInterval(f,1000);
            function f(){
                count++;
                let h = document.querySelector("h1")
                h.innerText =count;
            }

            let num = 0;
            //匿名开启定时器
            let timer =setInterval(function () {
                num++;
                let h = document.querySelector("h2")
                h.innerText=num;
                if (num==50){
                    //停止定时器
                    clearInterval(timer);
                }
            },100)

            //开启只执行一次的定时器
            setTimeout(function () {
                alert("时间到")
            },3000);
        </script>
    </body>
</html>